Alkali metal salts of cellulose sulfate are well known water soluble polymers useful in applications such as food additives (thickeners, enhancers), in textiles (sizings), in photographic coatings, in enhanced oil recovery (viscosity modifiers) and lowering cholesterol levels in humans. One known method of preparing alkali metal salts of cellulose sulfate is by reacting cellulose with a large excess amount of sulfuric acid (8-10 parts/part cellulose) in an inert diluent such as toluene with isopropanol to modify the reactivity of the sulfuric acid. The sulfonated cellulose is then exposed to a solution containing alkali metal ions to form the salt of the sulfate.
A drawback to this method, known as heterogeneous esterification is the cellulose does not readily dissolve in sulfuric acid and is thus non-uniformly substituted with sulfate ester groups. Another drawback is that a low degree of substitution occurs (&lt;1.0) of sulfate ester groups on the cellulose polymer backbone. Further, use of the large excess of sulfuric acid is accompanied by a substantial reduction in molecular weight of the cellulose due to degradation by the sulfuric acid.
Another method of preparing cellulose sulfate esters includes the steps of reacting cellulose in a fibrous state in dimethylformamide (DMF) with a preformed 1:1 complex of sulfur trioxide:dimethylformamide (SO.sub.3 :DMF). In another method cellulose may be reacted with dinitrogen tetroxide (N.sub.2 O.sub.4) in DMF solvent to form a soluble cellulose nitrite ester which is then reacted with the SO.sub.3 :DMF complex to form cellulose sulfate esters.
A drawback to both these methods is that SO.sub.3 is very difficult to handle as it vigorously fumes in the presence of air and reacts violently when contacted with water and other organic solvents.
Another method of producing cellulose sulfate involves preparing a slurry of cellulose in DMF to which N.sub.2 O.sub.4 is added to form cellulose nitrite ester. The cellulose nitrite ester can then be easily converted to water soluble cellulose sulfate esters by reaction with a mixture of sulfuric acid-acetic anhydride. A drawback to this method is that it uses large amounts of different chemicals which is not economical in terms of cost and handling of the reagents.
It is therefore advantageous to provide a method of producing alkali metal salts of cellulose sulfate which is both economical, provides a high degree of substitution and does not result in significant degradation of the cellulose.